Technologies from the Field

THE USE OF MAGNETIC RESONANCE
SPECTROSCOPY AND MAGNETIC RESONANCE IMAGING IN ALCOHOL RESEARCH

Bonnie
J. Nagel, Ph.D., and Christopher D. Kroenke, Ph.D.

BONNIE
J. NAGEL, PH.D., is an assistant professor in the Department of Psychiatry and
Behavioral Neuroscience at the Oregon Health & Science University, Portland,
Oregon.

CHRISTOPHER D. KROENKE, PH.D., is an assistant professor in the
Department of Behavioral Neuroscience and an assistant scientist in the Advanced
Imaging Research Center and Oregon National Primate Research Center, Oregon Health
& Science University, Portland, Oregon.

The recent emergence of magnetic resonance (MR)- based neuroimaging techniques
has dramatically improved researchers’ ability to understand the neuropathology
of alcoholism. These techniques range from those that directly monitor the metabolism
and the biochemical and physiological effects (i.e., the pharmacodynamics) of
alcohol within the brain to techniques that examine the impact of heavy alcohol
use on brain structure and function.

In general, MR-based techniques measure
electromagnetic signals (the same type of signals detected by a radio antenna)
generated by nuclei of endogenous molecules in the body of a person placed in
a powerful magnet field. When influenced by a magnet, tissue itself transiently
becomes magnetic. In part, this is because of the properties of atomic nuclei.
Different MR-based techniques have been developed to utilize nuclear magnetism
induced in tissue to generate images of internal structure. The most commonly
used MR imaging (MRI) techniques rely on signals derived from hydrogen nuclei
in water, which is by far the most concentrated molecular species in the body.
The physical properties of water molecules vary from one region of tissue to another,
and this influences the nuclear magnetism generated by water hydrogen nuclei.
As a result, MRI can differentiate regions in soft tissue at a high level of detail.
A second approach—MR spectroscopy (MRS)— uses the same strategy to
detect electromagnetic signals, but they are derived from nuclei of atoms (hydrogen
as well as some other atoms) on molecules other than water, such as lipids, amino
acids, or even alcohol (i.e., ethanol). The resulting data on the molecule(s)
under investigation can provide detailed information about the metabolic activity
of various tissues, including the brain. The main advantage of MR-based techniques
is that they do not expose the subject to radioactive tracers and therefore can
be used repeatedly in the same subject, allowing researchers to track metabolic
or structural changes over time.

This article briefly summarizes how these
techniques may be used to characterize the effects of alcohol dependence on the
brain.

Direct Measurement of Alcohol in the Brain

As indicated
above, MRS is the most direct MR-based technique for studying alcohol in the brain.
This approach has been used to characterize alcohol pharmacodynamics in rodents
(Adalsteinsson et al. 2006), humans (Hetherington et al. 1999), and nonhuman primates
(see figure 4). However, it is unclear whether this technique can measure ethanol
concentrations in the brain accurately because in several quantitative studies,
MRS-based estimates of alcohol concentrations in the brain were reported to be
lower than expected, based on blood alcohol concentration measurements (Chiu et
al. 2004; Kaufman et al. 1994, 1996; Moxon et al. 1991). To explain this observation,
Moxon and colleagues (1991) have argued that the hydrogen nuclei of some of the
ethanol molecules (i.e., of those that are bound to membranes) possess certain
characteristics1 (1Moxon and colleagues (1991) suggested
that the spin–spin relaxation time constants (T2) of the 1H nuclei of membrane-bound
ethanol is so short that these nuclei cannot be detected by MRS.) that make them
undetectable by in vivo MRS. This phenomenon may be relevant for alcoholism research
because some evidence suggests that the amplitude of the MRS signal for alcohol
that can be observed following a given alcohol dose changes with repeated alcohol
exposure (Govendaraju et al. 1997; Moxon et al. 1991) and that this change potentially
is related to the development of tolerance (Kaufman et al. 1994, 1996). To clarify
the potential link between changes in alcohol MRS intensity and alcohol exposure,
it is therefore important to determine whether alcohol truly is partially “invisible”
to MRS in the brain (Chiu et al. 2004) and whether brain alcohol concentrations
may be accurately measured by MRS if the relevant characteristics of the hydrogen
nuclei are carefully determined (Hetherington et al. 1999; Sammi et al. 2000).

The effects of chronic alcohol exposure on the brain and its neurochemistry
also can be assessed through MRS measurements of endogenous compounds naturally
produced in the body. One of these is a compound called N-acetylaspartate
(NAA), which is one of the most abundant molecules in neurons and usually provides
a large signal in brain MRS measurements (see figure 4C) (Mason et al. 2005, 2006).
NAA levels are reduced in numerous neuropathological conditions. According to
one report, chronic heavy drinkers also exhibit reduced intensity of the NAA signal
compared with control subjects (Mason et al. 2005), with larger effects seen in
females than in males. Although this observation is consistent with several potential
explanations (Mason et al. 2006), one popular interpretation of reduced NAA levels
in drinkers is that it reflects some form of neuronal loss or pathology.

Figure
4. Magnetic resonance spectroscopy (MRS) of ethanol in the nonhuman primate
brain. A) MRS data acquired from a rhesus macaque over the course
of a 2-g/kg intravenous infusion of alcohol. The image shows a lengthwise cut
through the brain, with the white rectangle delineating the area that was used
for the analysis. B) Specifically, spectra were acquired from
each of the 24 regions delineated by the grid, which is projected on a horizontal
image of the brain at the position indicated by the red dashed line in panel A.
C) An example of an MRS spectrum obtained from the highlighted (yellow)
brain region in B obtained prior to alcohol infusion (red trace) and again following
alcohol infusion (black trace). The spectrum shows the ethanol peak as well as
peaks for other endogenous compounds, such as N-acetylaspartate (NAA),
cholinecontaining compounds (Cho), and creatine (tCr). D) The
alcohol signal is quantified versus time.

Assessing
Structural Changes Associated With Alcohol Use

It is well known that chronic
alcohol use is associated with gross anatomical changes in the brain. Structural
MRI analyses in particular have greatly enhanced our understanding of these alcohol-related
changes. Based on differences in certain properties (i.e., spin relaxation properties)
of water molecules in various types of brain tissue, researchers can classify
individual volume elements (i.e., voxels) on the MRI images into gray matter,
white matter, and cerebral spinal fluid (see figure 5). Using these methods, several
studies have revealed alcohol-related reductions in gross brain tissue volumes
(Kril and Halliday 1999). In addition, the high resolution of MRI has facilitated
the measurement of smaller structures in the brain, and studies have shown reductions
in the volume of various brain structures, including the hippocampus (Agartz et
al. 1999; Beresford et al. 2006), corpus callosum (Hommer et al. 1996; Pfefferbaum
et al. 1996), striatum (Sullivan et al. 2005), and cerebellum (Shear et al. 1996),
in people with alcohol use disorders. Because MRI analyses can be performed repeatedly
in the same subject, the technique allows for longitudinal followup of alcohol-dependent
people after treatment. Such studies have suggested that structural recovery in
the brain may be possible in people achieving sustained abstinence (Cardenas et
al. 2007; Shear et al. 1994).

NOTE: The image
is a segmented skull-stripped T1-weighted anatomical image. This automated segmentation
was performed using Oxford Centre for Function Imaging of the Brain’s (FMRIB)
Automated Segmentation Tool (FAST).

An additional MRI-based
technique, termed diffusion tensor imaging (DTI), allows investigators to study
brain pathology on a microstructural scale. This technique exploits the passive
movement (i.e., diffusion) of water molecules within a tissue or structure. For
example, many neurons have one long extension (i.e., the axon) that connects to
other nerve cells and transmits signals to them. This axon typically is surrounded
by a sheath made up of a molecule called myelin. Furthermore, the myelincovered
axons of several nerve cells may be held together in axon bundles. Because the
myelin gives these bundles a whitish appearance, brain areas containing many of
these bundles also are referred to as white matter (as opposed to gray matter,
which is made up of nerve cell bodies). In healthy white matter, myelinated axon
bundles selectively restrict water diffusion, so that the water molecules tend
to move along the white matter tracts but not in a perpendicular direction. As
a result, diffusion is orientation dependent, or anisotropic. DTI measurements
have identified reduced diffusion anisotropy within the frontal white matter of
chronic alcoholics (Harris et al. 2008; Pfefferbaum et al. 2005, 2006), which
is interpreted as a manifestation of alcohol-related white matter damage. This
interpretation is further supported by findings that deficits in diffusion anisotropy
are associated with impairments in working memory (Pfefferbaum et al. 2000).

Functional
MRI Studies Related to Alcohol Dependence

Functional MRI (fMRI) is a powerful
tool that allows researchers to assess blood flow, and thereby brain function,
in a specific brain region. In general, blood flow is increased in brain regions
that are active at a given time and decreased in inactive regions or areas affected
by illness or damage. One way of assessing blood flow is by using positron emission
tomography (PET), which uses radioactive tracer molecules to track blood flow.
(For more information, see the article by Thanos et al., pp. 233–237.) However,
the use of radioactive compounds is an obvious disadvantage of that approach,
which can be avoided by fMRI. It is based on the observation that blood supplies
oxygen to active neurons at a greater rate than to inactive neurons. The increased
delivery of oxygen to a specific brain region leads to a magnetic signal variation
that can be detected using an MRI scanner. By taking rapid sequences of images
and tracking these variations, researchers can examine brain functioning during
a variety of cognitive and behavioral tests.

fMRI has furthered alcohol
research by allowing investigation of the neural circuits that are impacted by
alcohol use. For example, fMRI has revealed abnormal responses in the frontal
lobe during verbal and spatial working memory tasks in alcoholics (Desmond et
al. 2003; Tapert et al. 2001). In addition, it has enriched researchers’
understanding of the course of alcohol abuse, dependence, and recovery by allowing
repeated studies at various points during the course of the disease. However,
beyond detecting such functional abnormalities in brain response associated with
cognitive tasks, fMRI has tremendously helped scientists identify the neural substrates
of alcohol dependence itself. Thus, fMRI studies have elucidated the neural substrates
of alcohol craving (Park et al. 2007). Another fMRI study of alcohol cue–related
reactivity demonstrated increased reward-based activity in response to alcohol
cues in a brain region called the ventral striatum, whereas non–alcoholrelated
rewards elicited a reduced brain response (Wrase et al. 2007). Abnormal brain
responses in these regions have been associated with susceptibility to relapse
(Sinha et al. 2007), and pharmacological treatments of alcoholism have shown to
reduce abnormalities in alcohol cue–related responding in the ventral striatum
(Myrick et al. 2008).

Conclusions

Different MR-based technologies
have allowed researchers to monitor alcohol levels in the brain, identify alcohol-induced
structural changes in the brain, and study the impact of alcohol on brain function.
To date, most of these studies have been conducted in human subjects. As described
in the following article by Boudreau and colleagues, recent technological advances
have allowed the application of these approaches also for studying various aspects
of alcohol dependence in mouse models.